In the business of developing and manufacturing paper towels for the consumer market, it is a continual objective to improve the absorbent characteristics of the product. For cleaning up some spills, the consumer needs high absorbent capacity. For some uses, consumers want a fast rate of absorbency. For other uses, a combination of high absorbent capacity and fast absorbent rate is desired. At the same time, constraints on achieving this objective include the need to maintain or reduce costs in order to provide the consumer with the highest possible value, which in part means minimizing the amount of fiber in the product.
It has now been discovered that the absorbent characteristics of multi-ply paper towel products can be improved by providing an inter-ply, wet-collapse resistant open channel structure within the product as well as providing an intra-ply pore structure that is conducive to good absorbent characteristics. In addition to the absorbent characteristics provided by the fibers and the intra-ply fiber network pore structure, the inter-ply channels also contribute to absorbency by readily wicking away liquid and rapidly distributing the liquid to other areas of the product that are not yet saturated, as well as providing reservoirs for holding absorbed liquid. This wet-collapse resistant open channel structure can take the form of relatively straight and long channels or it can the form of a more tortuous path imparted to the surface of the plies by various papermaking fabric contours and embossing patterns. One convenient method of creating such channels is to ply together multiple low basis weight uncreped throughdried plies which have a highly contoured surface of multiple ridges and which inherently have wet-collapse resistance due to the manner in which they are formed, processed and dried. However, other means of providing suitable wet-collapse resistance include the use of appropriate fibers, including synthetic fibers, crimped fibers and pre-crosslinked fibers, optionally in conjunction with suitable converting operations such as heated embossing to shape the structure, or using any means of drying a wet sheet into a three-dimensional configuration during manufacturing or converting of the paper such that the paper retains the three-dimensional shape when wetted with water.
Hence, in one aspect, the invention resides in a multi-ply paper sheet, such as a paper towel, having two or more plies, said sheet having a Vertical Absorbent Capacity of about 8.0 grams of water or greater per gram of fiber and a Wet Out Time of 2.3 seconds or less.
In another aspect, the invention resides in a multi-ply paper towel having two or more throughdried plies, including uncreped throughdried plies, said towel having a Vertical Absorbent Capacity of about 8.0 grams of water or greater per gram of fiber and a Wet Out Time of 2.3 seconds or less.
In the various aspects of this invention, the Vertical Absorbent Capacity can be about 8.0 grams of water or greater per gram of fiber, more specifically about 9.0 grams of water or greater per gram of fiber, more specifically about 10.0 grams of water or greater per gram of fiber, more specifically from about 8.0 to about 16.0 grams of water per gram of fiber, more specifically from about 9.0 to about 15.0 grams of water per gram of fiber, and still more specifically from about 10.0 to about 15.0 grams of water per gram of fiber.
In the various aspects of the invention, the Wet Out Time can be about 2.3 seconds or less, more specifically about 2.0 seconds or less, more specifically about 1.5 seconds or less, more specifically from about 0.5 to 2.3 seconds, more specifically from about 0.5 to about 2.0 seconds, more specifically from about 0.5 to about 1.5 seconds, and still more specifically from about 0.9 to about 1.5 seconds.
Factors which reduce the Wet Out Time include: enlarging the pore size of the plies, which can be achieved by using more three-dimensional throughdrying fabrics as described herein; reducing the basis weight of the plies; fiber selection; decreasing the amount of refining of the fibers; selecting hydrophilic chemical additives; increasing the amount of surfactant in the furnish.
The number of plies in the product can be two, three, four, five or more. For economy, two-ply and three-ply products are advantageous. The various plies within any given multi-ply sheet can be the same or different. By way of example, the various plies can contain different fibers, different chemicals, different basis weights, or be made differently to impart different topography. Different processes include throughdrying, creped or uncreped, wet-pressing or modified wet-pressing. Wet molded throughdried plies, such as uncreped throughdried plies, have been found to be particularly advantageous because of their wet resiliency and three-dimensional topography.
The fibers used to form the plies of the products of this invention can be substantially entirely hardwood kraft or softwood kraft fibers. However, other fibers can also be used for part of the furnish, such as mechanical pulp fibers, bleached chemithermomechanical pulp (BCTMP) fibers, synthetic fibers, pre-crosslinked fibers, non-woody plant fibers, and the like. More specifically, the fibers can be from about 50 to about 100 percent softwood kraft fibers, more specifically from about 60 to about 100 percent softwood kraft fibers, still more specifically from about 70 to about 100 percent softwood kraft fibers, still more specifically from about 80 to about 100 percent softwood kraft fibers, and still more specifically from about 90 to about 100 percent softwood kraft fibers. While not being bound to any particular theory, it is believed that throughdried sheets formed primarily from virgin softwood kraft fibers have a particularly desirable internal pore structure that synergistically interacts with the wet-collapse resistant channels to provide the improved absorbent properties observed.
The basis weight of the plies used to produce the multi-ply products of this invention can be about 40 gsm or less, more specifically about 30 gsm or less, more specifically about 20 gsm or less, still more specifically from about 10 to about 20 gsm, and still more specifically from about 10 to about 15 gsm.
As used herein, xe2x80x9cVertical Absorbent Capacityxe2x80x9d is a measure of the amount of water absorbed by the paper towel product, expressed as grams of water absorbed per gram of fiber (dry weight) in the product. In particular, the Vertical Absorbent Capacity is determined by cutting a sheet of the product to be tested into a square measuring 100 millimeters by 100 millimeters (xc2x11 mm.) The specimen is weighed to the nearest 0.01 gram and the value is recorded as the xe2x80x9cDry Weightxe2x80x9d. The specimen is placed into a dish of water and soaked in the water for 3 minutes (xc2x15 seconds). At the end of the soaking time, the specimen is removed from the water and hung from one corner in a clamping device such that the opposite corner is lower than the rest of the specimen. The specimen is allowed to drain for 3 minutes (xc2x15 seconds). At the end of the specimen draining time, the specimen is removed by holding a weighing dish under the specimen and releasing it from the clamping device. The wet specimen is then weighed to the nearest 0.01 gram and the value recorded as the xe2x80x9cWet Weightxe2x80x9d. The Vertical Absorbent Capacity=(Wet Weightxe2x80x94Dry Weight)/Dry Weight. At least five (5) replicate measurements are made on the same sample to yield an average Vertical Absorbent Capacity value.
The absorbent capacity of the multi-ply products of this invention, on an equivalent pore volume basis as measured by capillary suction, is such that about 50 percent or more of the total pore volume resides in pores having a radius of 250 micrometers or greater.
As used herein, xe2x80x9cWet Out Timexe2x80x9d is a measure of how fast the paper towel product absorbs water and reaches its absorbent capacity, expressed in seconds. In particular, the Wet Out Time is determined by selecting and cutting 20 representative product specimen sheets into squares measuring 63 millimeters by 63 millimeters (xc2x13 mm.). The resulting pad of 20 product sheets is stapled together across each corner of the specimen pad just far enough from the edges to hold the staples. The staples should be oriented diagonally across each corner and should not wrap around the edges of the test specimen. With the staple points facing down, the specimen is held horizontally over a pan of water approximately 25 millimeters from the surface of the water. The specimen is dropped flat onto the surface of the water and the time for the specimen to become visually completely saturated with water is recorded. This time, measured to the nearest 0.1 second, is the Wet Out Time for the product. At least five (5) replicate measurements are made on the same sample to yield an average Wet Out Time value.